1. Field of the Invention
This invention relates to an electrophotographic copying method and apparatus. More specifically, it relates to an electrophotographic copying method and apparatus for forming outline images corresponding to peripheral outlines of document's images.
2. Description of the Prior Art
Generally speaking, the peripheral outline of an image is in practice full of necessary information thereabout and represents sufficiently the characterizing features of the given image, thus playing among others a most important role in the judgement of the latter.
The so-called outline image forming means is adapted such that a peripheral outline is picked up from a generally full and positive documentary image and is devoid of intermediate tones or reversely, solid representations, thus being most effective for the identification of the practical image and for pattern recognition purpose thereof.
For example, complex color image patterns can be obtained by such outline image forming means in a manner of forming by execution of successive copying operations a blank pattern encircled by a color outline, or preparing a blank pattern for later producing differently colored local image areas contained therein.
It should be noted that in the name of the present assignee company per se, it has already been proposed to realize an outline image forming process, wherein, in the mono-component type toner developing method using conductive toner to develop static latent images, a DC bias voltage is applied between the material to be subjected to developing and the carrier for the conductive toner, said voltage being at a medium potential ranged between maximum and minimum surface potentials of said material and having an opposite polarity to that of the static latent image charge, thereby to extract the outline marginal configuration only from the latent image on said material to be developed (refer to Japanese Patent Laying-Open Gazette No. 134635/1976).
It should be further noted, however, that there is a considerable drawback in the above mentioned prior art process. In this proposed process, the developed marginal outline image is only of negative one, because the marginal outline of the static latent image is also negative and the conductive toner will be deposited onto substantive part of the latent image devoid of the marginal outline thereof, and indeed, by virtue of higher potential difference at the substantive part of the latent image on the drum. In practice, however, a desired marginal outline should preferably be in black and thus positive. Therefore, the thus formed negative outline image must generally be subjected to a further reproducing step relying upon the reverse development principle, which represents naturally a considerable and troublesome drawback inherent in the above mentioned known process.
Under the circumstances, the inventors of the present invention have proposed electrophotographic copying methods in which a second charger is provided between an exposure device and a developing device and an outline image is obtained by operation of the second charger, as disclosed in the U.S. patent application Ser. Nos. 16,716 (filed Feb. 19, 1987), 16,717 (filed Feb. 19, 1987) and 58,266 (filed May 21, 1987). In those methods, a normal copy image can be obtained if the second charger is turned off. On the other hand, if copy operation is performed with the second charger being turned on, an electrostatic latent image formed on the photoconductor is processed as an outline image prior to a process of the developing device and thus the outline image can be obtained.
In general, developer obtained by mixing carrier and toner at a fixed ratio is used to develop an electrostatic latent image. In order to obtain an image of good quality, it is necessary to supply toner in compensation for an amount of toner consumed by development.
Consequently, the above mentioned proposed methods adopt a control method for toner supply to a developer tank, as described below. A toner density sensor is provided in a cleaning device or the like adjacent to a central portion of a photoconductor drum and a reference latent image at a given potential is formed and developed on the surface of the photoconductor drum prior to formation of an image of an original. Then, a toner density corresponding to an amount of toner adhering to the reference latent image is detected by the toner density sensor and a toner supply roller or the like is driven based on the detected value, so that the toner density of the developer in the developer tank can be maintained constant.
However, if the above mentioned toner density control method is used in a copying apparatus capable of forming an outline image using the above mentioned second charger, the following problems would occur.
In the above mentioned toner density control method, it is necessary to form a reference latent image at a given potential on the surface of the photoconductor drum and if the reference latent image is formed at the time of copy operation in an outline image forming mode, the potential of the reference latent image is lowered except for that in the outline portions due to the charging effect of the second charging device. As a result, if the reference latent image is developed, little amount of toner adheres to the image (except for the outline portions) and the toner density sensor erroneously detects a low toner density irrespective of a real toner density, thereby to instruct unnecessary supply of toner. If such unnecessary supply of toner is continued, the developer in the tank has an excessive amount of toner, which would cause stains on an image or fine splashes of toner to soil components around the developing device.